The term “cataract” refers to a turbidity of the lens of the eye, i.e. a decrease in its transparency. Certain forms of cataracts develop relatively quickly, but the great majority develop over a period of several decades. Severe forms of cataracts therefore occur principally in older people. Surgical methods for the removal of cataracts have been known for some time, and represent routine procedures. The lens is usually replaced in this context by a plastic lens (intraocular lens, IOL).
In almost all forms of cataracts, the impairment in vision is due not to increasing opacity of the lens and thus to increased light absorption, but instead to a structural change in the lens that results in increased light scattering. This light scattering causes a decrease in the contrast of the field of vision.
Cataract surgery is not entirely risk-free. Because cataracts develop slowly, the question as to the correct time for an operation is therefore in some circumstances difficult to answer.
In a cataract operation, the lens body is as a rule disintegrated, removed, and replaced by a plastic lens. The lens capsule remains in the eye. After lens removal, the surgeon attempts to remove remaining lens residues as completely as possible by means of the aforementioned capsule polishing. At present, the posterior lens capsule is cleaned largely by feel, and in accordance with the subjective assessment and experience of the surgeon.
For stability reasons, the posterior capsule of the lens of the eye is not removed in the cataract procedure. Serious complications can otherwise occur. On the other hand, fibrous proliferation of certain cells (“capsular fibrosis”) can occur on the posterior lens capsule that remains after the operation, causing turbidity to reoccur. Visual impairments can also, however, arise principally because lens residues or very thin membranes remain behind on the posterior lens capsule. An “after-cataract” of this kind forms in up to 30% of cases after a cataract operation. The exact causes are not completely known; this is also due to an absence of objective measurement methods, for example to check the surgical outcome. Further operations or laser treatments are necessary in order to remove the after-cataract.
Lens residues are transparent media, however, and are very difficult to detect. Capsule polishing moreover represents a very severe stress for the capsule, which can thereby be damaged. It is therefore of interest to detect remaining lens residues, especially intraoperatively, in order to avoid a possible after-cataract.
No possibility so far exists for intra- or postoperatively sensing and quantifying the state of the posterior lens capsule during the operation. Maximally complete removal of lens residues, membranes, etc., also called “lens polishing,” is usually performed exclusively visually, and can therefore lead to the aforementioned residual risk.
In light of this, a need therefore exists for improved surgical microscope systems that offer corresponding diagnostic capabilities.
Optical coherence tomography (OCT) represents a diagnostically valuable optical image-producing method especially in biomedical optics and in medicine. A comprehensive review of the existing art is provided, for example, in W. Drexler and J. G. Fujimoto (eds.), “Optical Coherence Tomography. Technology and Applications,” Springer, 2008.
Optical coherence tomography makes possible high-resolution cross-sectional depictions of the internal microstructure of biological tissue by measuring light that is reflected at interfaces at different depths. Unlike methods such as ultrasonic tomography, optical coherence tomography is non-contact and therefore does not stress the patient. Corresponding structures can be sensed in real time and at a resolution of 1 to 15 μm, i.e. approximately two orders of magnitude higher than with ultrasound.
In optical coherence tomography, low-coherence light of a corresponding light source is split at a beam splitter of an interferometer, e.g. a Michelson interferometer, into a reference arm and a measurement arm. Light of the reference arm is reflected at a corresponding mirror, and light of the measurement arm at a structure of the object being investigated.
After reflection at the respective surfaces, the signals of the reference arm and measurement arm are overlaid. Interference between the signals of the two arms yields a pattern from which the relative optical path length can be derived. A corresponding depth profile measurement is also referred to as an “axial scan” (A-scan). As a rule an optical coherence tomograph has a scanning device, so that a corresponding sample can also be scanned transversely in one or two directions. The two-dimensional scan that is obtained extends parallel to the axial scan as a perpendicular section through the eye, and is called the B-scan. The three-dimensional scan that is obtained is called the C-scan.
The interior of the eye is substantially transparent, and in the healthy state therefore transmits light of corresponding wavelengths with only minimal optical attenuation and scattering. Optical access both to the anterior segment and to the ocular fundus is therefore generally good. Ophthalmologic imaging, in particular of the retina, was therefore one of the first areas of application of optical coherence tomography. Optical coherence tomography makes possible, for example, early diagnosis of retinopathies and macular degeneration.
Specifically adapted coherence tomographs are used to investigate the anterior region of the eye. They must ensure a sufficiently high scanning rate (e.g. 4,000 A-scans per second) that even relatively large-area structures can be sensed within an acceptable time. Light sources having suitable wavelengths must also be used, since the healthy eye is almost transparent to the wavelengths (for example, 830 nm) usually utilized for coherence tomography of the ocular fundus. Light having a wavelength of, for example, 1310 nm is therefore used for corneal investigation.
The areas of application of optical coherence tomography in the anterior region of the eye encompass in particular measurement of corneal thickness, evaluation of narrow-angle glaucoma, and measurement of the anterior chamber of the eye. Reference is made in this context to the article “Interoperative Two-Dimensional Optical Coherence Tomography as a New Tool for Anterior Segment Surgery,” Gerd Geerling et al. (reprinted), ArchOphthalmol Vol. 123, February 2005.
In systems of this kind, lateral scanning can occur in the form of a so-called sector scan (diverging), a right-angle or telecentric scan (using a parallel-shifted measurement arm), or in converging fashion.